The very rapid growth in data traffic, especially in wide-area technology, requires an increase in the transmission capacity of present and future transmission systems. To that end, optical transmission systems are operated with wavelength-division multiplexing (WDM), in which optical WDM signals are transmitted in individual channels, or WDM channels. This technology now constitutes the preferred solution for implementing the required transmission-capacity increases.
For the error-free transmission of WDM signals, it is necessary for the reception unit of the optical transmission system to have channel levels, or WDM channel levels, with a constant value, so that as the WDM channel number increases, especially when a plurality of WDM signals are being transmitted, the total power to be transmitted in the optical fiber increases. Raising the total power in the optical fiber, however, requires optical amplifier arrangements that have high output powers. Besides the high output power, it is necessary, especially for an optical amplifier arrangement, to have a flat gain spectrum over a wide wavelength range. Inline amplifier arrangements, in particular, require low-noise production of the necessary amplification power and compensation for the dynamic gain tilt, with the aid of which it is possible to compensate for different path attenuations.
It is further necessary for such optical inline amplifier arrangements to have an intermediate pick-off for inserting dispersion compensation units or optical filters to branch off or insert optical WDM signals, or WDM channels.
To produce such optical amplifiers, or amplifier arrangements, that have high output powers, essentially three approaches are to date known, or have been proposed. One of the approaches is based on the conventional single-mode technique. The requisite high pump powers of the optical amplifier are provided by coupling together the output signal from a plurality of pump laser diodes having single-mode fiber outputs, the pump wavelengths being in the wavelength range around 1480 nm. Polarization couplers and band-separating filters, or wavelength-selective multiplexers, are used as the coupling elements in this case. The pump power generated in this way is injected into the active fiber, for example an erbium-doped optical fiber, via a wavelength-selective multiplexer. See Y. Tasiro et al. xe2x80x9c1.5 W Erbium Doped Fiber Amplifier Pumped by the Wavelength Division-Multiplexed 1480 nm Laser Diodes with Fiber Bragg Gratingxe2x80x9d, Technical Digest of the Conference on Optical Amplifiers and their Applications (1998), WC2-1, pages 213 to 215.
A further approach involves generating the high pump power with the aid of a Raman pump laser. See G. R. Jacobotic-Weselka et al.: xe2x80x9cA 5.5-W Single-Stage-Single-Pumped Erbium-Doped Fiber Amplifier at 1550 nmxe2x80x9d, Technical Digest of the Conference on Optical Amplifiers and their Applications (1997), PD3, pages 1-4. In this case, individual semiconductor laser diodes or linear diode arrays in the wavelength window around 900 nm, with the aid of which high output powers can be generated cost-effectively, are used as primary radiation sources. Their output signal is not injected into a single-mode fiber, but rather fed in free-beam fashion or via a multimode fiber into the inner cladding of a special active optical fiber having a double cladding. The inner cladding guides the pump radiation in multimode fashion, and therefore permits simple and efficient injection of the pump radiation. The core of the active optical fiber is doped with dopant ions. These absorb the pump radiation and emit at longer wavelengths, typically around 1060 or 1100 nm, so that optical signals in this wavelength range experience amplification. In this case, laser oscillations at the emission wavelengths are generated with the aid of a resonator. Since the active fiber guides the emission radiation in a single-mode fashion, it can be processed further in single-mode fiber technology. Raman cascade lasers are used for converting the pump radiation into the wavelength ranges needed for pumping the optical amplifier arrangement.
The described multimode technique can be used for directly pumping the active optical fiber of an optical amplifier. In such an embodiment, the active fiber of the optical amplifier itself has an inner cladding, which guides the pump radiation in multimode fashion and permits simple injection of the pump radiation. Besides the dopant ions needed for the amplification process, there are further ions in the core. Their purpose is to absorb the pump radiation and forward it to the amplifier ions by non-radiative transfer processes.
An aspect common to the described embodiments of optical amplifiers with high output power is that they have to date been suitable essentially for the construction of complete booster amplifiers. Such booster amplifiers have total input powers around 0 dBm and a total output power of from approximately 27 dBm to over 33 dBm.
A method, and an amplifier arrangement, for stepwise upgrading of the output power of optical amplifiers is furthermore known, in which an amplifier arrangement, produced in conventional single-mode technology and having non-equipped pump inputs, as well as additional external pump sources are provided. In this approach, the stepwise upgradeable optical amplifier arrangement internally has all the components needed for normal operation, i.e. a long active fiber, pump-WDM couplers etc., although further external pump lasers or pump sources may be connected to the existing pump inputs for increasing the output power of the amplifier arrangement. In the case of low channel numbers, or few WDM signals to be transmitted, the requisite output power is generated by the optical amplifier arrangement without connecting in additional external pump sources. A rise in the output power of the optical amplifier arrangement, needed for example to double the WDM channel number, is possible by injecting externally generated pump radiation, or pump signals, into the active fiber via the pump inputs, or via the outwardly routed input branches of the pump-WDM coupler that is already present internally.
When optical transmission systems, or optical transmission paths, are first commissioned, it is customary not to utilize the full channel number, or WDM channel number, i.e. only a few optical WDM signals, or WDM channels are initially transmitted via the optical transmission path. It should therefore be possible for optical amplifier arrangements to be modularly upgradeable in terms of their output power, and hence permit a stepwise increase in the optical WDM channel number. If the channel number, or the number of optical WDM signals, is still low when the optical transmission path is first operated, then cost-effective optical amplifier arrangements can be used. When the demand for transmission capacity increases, i.e. the WDM channel number increases, it is necessary for the optical amplifier arrangement to be uprated cost-effectively in stages, in order to deliver the output powers respectively needed for the higher channel number.
An advantage of the invention is to provide a cascadable optical amplifier arrangement which permits a stepwise increase of the output power of a base amplifier arrangement that is constructed modularly and in single-mode technology.
In an embodiment, a cascadable optical amplifier arrangement is provided having a modular base amplifier arrangement (BVA) that is constructed in single-mode technology and has at least one amplifier stage (VS1 to VS4), and having a high-power amplifier stage (HVS1) that can be connected to the at least one amplifier stage (VS4) of the base amplifier arrangement (BVA) and has its own active fiber (AF) and at least one pump signal source (PSQ1, PSQ2).
An advantage of the cascadable optical amplifier arrangement according to the invention is that a base amplifier arrangement, which is constructed modularly and in single-mode technology and has at least one amplifier stage, is provided with at least one high-power amplifier stage that can be connected to the at least one amplifier stage of the base amplifier arrangement and has its own active fiber as well as at least one pump signal source. The cascadable optical amplifier arrangement according to the invention, for power upgrading with at least one additional high-power amplifier stage, has the particular advantage over the known design with external pump sources that upgrading can be carried out in a plurality of stages, so that the currently and most cost-effectively implementable amplifier technology available can be used to produce a further high-power amplifier stage. By virtue of this, it is possible to use technologies for high output power that were not yet available when the base amplifier was commissioned. The base amplifier arrangement has the output power needed when the optical transmission path is constructed, as well as the amplifier components needed to produce this, which permits very cost-effective technical implementation of the base amplifier arrangement. For example, a network customer does not need to pay for the additional output power of the cascadable optical amplifier arrangement until a subsequent upgrade with the high-power amplifier stages according to the invention.
The cascadable optical amplifier arrangement according to the invention further makes it possible to retrofit optical transmission systems which were not originally intended for upgrading in terms of optical output power, or whose optical amplifiers have no facility for increasing the output power.
Advantageously, at least one further high-power amplifier stage (HVS2) can be connected to the high-power amplifier stage connected to the base amplifier arrangement. If such high-power amplifier stages are cascaded according to the invention, it is possible to increase the output power of the optical amplifier arrangement stepwise. The serially connected high-power amplifier stages have a comparatively low gain, which leads to a relatively simple technical structure. Because of said low gain, optical isolators need be used in the respective high-power amplifier stages only in exceptional cases, in particular if the existing base amplifier arrangement already has an optical isolator at its output.
Further, it is particularly advantageous if the high-power amplifier stages each have their own amplification control and/or power control, which may be produced both optoelectronically and purely optically. With the aid of the amplification control and/or power control, the serially connected, or cascaded, high-power amplifier stages can be matched to the requirements of the respective application site within the optical transmission network.
It is particularly advantageous if a filter serially connected upstream of the active fiber of the high-power amplifier stages is provided in order to level the gain spectrum of the optical signal to be amplified. The additional optical filter units needed for flattening the gain spectrum can be produced simply and cost-effectively, since they do not need to meet any special technical requirements. A flat gain spectrum of the high-power stage according to the invention can additionally be achieved by optimizing the amplifier properties, or the active fiber, of the high-power amplifier stages.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the figures.